<p>The passive-corrosion mechanism of passive film for steel rebar in a simulated concrete medium was investigated. The effects of Cl⁻ and SO₄²⁻ on the passivation films were analyzed by electrochemical methods and further verified by visually characterizing the structural changes of the passivated films in combination with XPS, SPM, and TEM techniques. The results revealed a significant discrepancy in defect density between the outer and inner layers of the passive film, while the impact of SO₄²⁻ concentrations on the film composition was relatively minimal. Additionally, the competitive adsorption mechanism between Cl⁻ and SO₄²⁻ was identified as the primary factor determining the protection of the passive film. At a concentration of 0.4&#xa0;M SO₄²⁻, the non-uniform corrosion dissolution at the junction between matrix and film accelerated the depassivation process of the steel reinforcement. However, the detachment of Cl⁻ occurred at a higher concentration of SO₄²⁻, which reduces the dissolution rate of the passive film and enhances its protective properties on the substrate.</p>